Transport control channel program message pairing

ABSTRACT

A method, apparatus, and computer program product for processing a chained-pair linked transport control channel program in an I/O processing system is provided. The method includes receiving a first command message at a control unit specifying that a device command word (DCW) list is encoded in a data message associated with the first command message as part of the chained-pair linked transport control channel program. The method further includes receiving a second command message chained-pair linked to the first command message, the second command message specifying data attributes associated with the DCW list. The method additionally includes extracting the DCW list from the data message in response to receiving the data message, and executing the DCW list.

BACKGROUND

1. Field of Invention

The present disclosure relates generally to input/output (I/O)processing, and in particular, to splitting a transport control channelprogram into linked pairs of command message blocks to pair commands andcustomer data in separate data messages in an I/O processing system.

2. Description of Background

Input/output (I/O) operations are used to transfer data between memoryand I/O devices of an I/O processing system. Specifically, data iswritten from memory to one or more I/O devices, and data is read fromone or more I/O devices to memory by executing I/O operations.

To facilitate processing of I/O operations, an I/O subsystem of the I/Oprocessing system is employed. The I/O subsystem is coupled to mainmemory and the I/O devices of the I/O processing system and directs theflow of information between memory and the I/O devices. One example ofan I/O subsystem is a channel subsystem. The channel subsystem useschannel paths as communications media. Each channel path includes achannel coupled to a control unit, the control unit being furthercoupled to one or more I/O devices.

The channel subsystem may employ channel command words (CCWs) totransfer data between the I/O devices and memory. A CCW specifies theI/O command to be executed. For commands initiating certain I/Ooperations, the CCW designates the memory area associated with theoperation, the action to be taken whenever a transfer to or from thearea is completed, and other options.

During I/O processing, a list of CCWs is fetched from memory by achannel. The channel parses each command from the list of CCWs andforwards a number of the commands, each command in its own entity, to acontrol unit coupled to the channel. The control unit then processes thecommands. The channel tracks the state of each command and controls whenthe next set of commands are to be sent to the control unit forprocessing. The channel ensures that each command is sent to the controlunit in its own entity. Further, the channel infers certain informationassociated with processing the response from the control unit for eachcommand. Performing I/O processing on a per CCW basis may involve alarge amount of processing overhead for the channel subsystem, as thechannels parse CCWs, track state information, and react to responsesfrom the control units.

SUMMARY

An exemplary embodiment includes a computer program product forprocessing a chained-pair linked transport control channel program at acontrol unit configured for communication with an input/output (I/O)subsystem in an I/O processing system. The computer program productincludes a tangible storage medium readable by a processing circuit andstoring instructions for execution by the processing circuit forperforming a method. The method includes receiving a first commandmessage at the control unit from the I/O subsystem, the first commandmessage specifying that a device command word (DCW) list is encoded in adata message associated with the first command message as part of thechained-pair linked transport control channel program. The methodfurther includes receiving a second command message at the control unitfrom the I/O subsystem chained-pair linked to the first command message,the second command message specifying data attributes associated withthe DCW list. The method also includes extracting the DCW list from thedata message in response to receiving the data message, and executingthe DCW list.

Another exemplary embodiment includes an apparatus for processing achained-pair linked transport control channel program at a control unitin an I/O processing system. The apparatus includes a control unitconfigured for communication with an I/O subsystem of the I/O processingsystem. The control unit receives a first command message from the I/Osubsystem, the first command message specifying that a DCW list isencoded in a data message associated with the first command message aspart of the chained-pair linked transport control channel program. Thecontrol unit receives a second command message from the I/O subsystemchained-pair linked to the first command message, the second commandmessage specifying data attributes associated with the DCW list. Thecontrol unit extracts the DCW list from the data message in response toreceiving the data message, and executes the DCW list.

A further exemplary embodiment includes a method for processing achained-pair linked transport control channel program at a control unitconfigured for communication with an I/O subsystem in an I/O processingsystem. The method includes receiving a first command message at thecontrol unit from the I/O subsystem, the first command messagespecifying that a DCW list is encoded in a data message associated withthe first command message as part of the chained-pair linked transportcontrol channel program. The method further includes receiving a secondcommand message at the control unit from the I/O subsystem chained-pairlinked to the first command message, the second command messagespecifying data attributes associated with the DCW list. The methodadditionally includes extracting the DCW list from the data message inresponse to receiving the data message, and executing the DCW list.

An additional exemplary embodiment is a computer program product forprocessing a chained-pair linked transport control channel program at achannel subsystem configured for communication with a control unit in anI/O processing system. The computer program product includes a tangiblestorage medium readable by a processing circuit and storing instructionsfor execution by the processing circuit for performing a method. Themethod includes configuring a first command message specifying that aDCW list is encoded in a data message associated with the first commandmessage as part of the chained-pair linked transport control channelprogram. The method further includes configuring a second commandmessage chained-pair linked to the first command message, the secondcommand message specifying data attributes associated with the DCW list.The method also includes transmitting the first and second commandmessages from the channel subsystem to the control unit, andtransmitting the DCW list from the channel subsystem to the control unitin the data message.

A further exemplary embodiment is an apparatus for processing achained-pair linked transport control channel program at a channelsubsystem configured for communication with a control unit in an I/Oprocessing system. The apparatus includes a channel subsystem configuredfor communication with a control unit of the I/O processing system. Thechannel subsystem configures a first command message specifying that aDCW list is encoded in a data message associated with the first commandmessage as part of the chained-pair linked transport control channelprogram. The channel subsystem configures a second command messagechained-pair linked to the first command message, the second commandmessage specifying data attributes associated with the DCW list. Thechannel subsystem transmitting the first and second command messagesfrom the channel subsystem to the control unit, and transmits the DCWlist from the channel subsystem to the control unit in the data message.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 depicts one embodiment of an I/O processing system incorporatingand using one or more aspects of the present invention;

FIG. 2 depicts one embodiment of a control unit and a channel subsystem,in accordance with an aspect of the present invention;

FIG. 3 depicts one embodiment of a chain-paired transport control word(TCW) channel program, in accordance with an aspect of the presentinvention;

FIG. 4 depicts one embodiment of a link protocol used to identify acompatible control unit of an I/O processing system, in accordance withan aspect of the present invention;

FIG. 5 depicts one embodiment of a request message of the link protocolof FIG. 4;

FIG. 6 depicts one embodiment of an accept message of the link protocolof FIG. 4;

FIG. 7 depicts one embodiment of a link protocol used to communicatebetween a channel and control unit to execute the chain-paired TCWchannel program of FIG. 3, in accordance with an aspect of the presentinvention;

FIG. 8 depicts one embodiment of a TCW in accordance with an aspect ofthe present invention;

FIG. 9 depicts one embodiment of a command message communicated from achannel subsystem to a control unit, in accordance with an aspect of thepresent invention;

FIG. 10 depicts one embodiment of a process for providing TCW channelprogram message pairing at a channel subsystem in accordance with anaspect of the present invention;

FIG. 11 depicts one embodiment of a process for providing TCW channelprogram message pairing at a control unit in accordance with an aspectof the present invention; and

FIG. 12 depicts one embodiment of an article of manufactureincorporating one or more aspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an aspect of the present invention, input/output(I/O) is facilitated, splitting a transport control channel program intolinked pairs of command message blocks to pair commands and customerdata in separate data messages. A transport control channel programfacilitates I/O processing by reducing communications between componentsof an I/O processing system used to perform the I/O processing. Forinstance, the number of exchanges and sequences between an I/Ocommunications adapter, such as a channel, and a control unit isreduced. This is accomplished through sending multiple commands and/ordata to the control unit grouped in blocks for execution at the controlunit rather than sending individual channel command words (CCWs).

Channel programs implemented with CCWs (also referred to as “CCW channelprograms”) involve a large degree of handshaking to perform tasks. Forexample, writing a 4 kilobyte block of data using a CCW channel programtypically requires an exchange to be opened, transmission of a defineextent command with data, transmission of a locate record command withdata, and transmission of a write command with data from the channel tothe control unit. The control unit typically responds by opening anexchange and sending a response to acknowledge the write command,sending a status message upon completing the write command, and closingthe exchange it opened. The channel may then respond by closing theexchange that it opened. Using a TCW channel program, a transportcommand control block (TCCB) can be sent from the channel to the controlunit as a block of commands, avoiding many of the messages between thechannel and the control unit that would otherwise be performed using aCCW channel program. For example, the TCW channel program can avoidopening an exchange to respond that the control unit received the writecommand. The cumulative effect over multiple command sequences canresult in a large time savings when running a TCW channel programinstead of a CCW channel program, and thus overall I/O processing systemthroughput is increased. In an exemplary embodiment, an I/O processingsystem can support CCW channel programs in command mode and TCW channelprograms in transport mode. Transport mode indicates that the channeltransports commands and data to the control unit without processing thecommands and data transported.

In an exemplary embodiment, the link protocol used for command modecommunications is FICON (Fibre Connectivity). Information regardingFICON is described in “Fibre Channel Single Byte Command Code Sets-3Mapping Protocol (FC-SB-3), T11/Project 1357-D/Rev. 1.6, INCITS (March2003), which is hereby incorporated herein by reference in its entirety.The link protocol used for transport mode communications may be, forinstance, Fibre Channel Protocol (FCP). In particular, three phases ofthe FCP link protocol can be used, allowing use of host bus adaptersthat support FCP to perform data transfers. FCP and its phases aredescribed further in “Information Technology—Fibre Channel Protocol forSCSI, Third Version (FCP-3),” T10 Project 1560-D, Revision 4, Sep. 13,2005, which is hereby incorporated herein by reference in its entirety.It will be understood that other versions of these protocols and/orsimilar protocols can be used within the scope of the invention.

A plurality of commands (e.g., device command words or “DCWs”) can beincluded in a TCCB, the contents of which are located via one or moreaddress references (indirect or direct) in a transport control word(TCW). In an exemplary embodiment, the TCW is sent from an operatingsystem (OS) or other application to the I/O communications adapter,which in turn forwards the TCCB in a command message to the control unitfor processing. The control unit processes each of the commands absent atracking of status relative to those individual commands by the I/Ocommunications adapter. The plurality of commands is also referred to asa channel program, which is parsed and executed on the control unitrather than the I/O communications adapter.

A single TCCB may be constrained in size as a function of a linkprotocol or buffer size constraints, which can in turn limit the numberof commands and/or amount of data associated with the TCCB. Some I/Ooperations can include a greater number of commands or volume of datathan may be incorporated in a single TCCB. In an exemplary embodiment,chain linking of multiple TCWs with associated TCCBs is employed tocreate larger TCW channel programs, allowing a single I/O operation tospan multiple TCWs and TCCBs. While linking multiple TCCBs and TCWstogether can provide support for larger TCW channel programs than canfit in a single TCCB, this approach may not be the most efficient insome scenarios. For example, if a single TCCB is limited to about 30commands, executing a program of 1000 commands may require chaining 34or more TCCBs and TCWs together to run the program, resulting in a largeamount of overhead. However, data information units (IUs) can containthousands of bytes per message. In an exemplary embodiment, pairs ofTCWs/TCCBs are chain linked together, where one TCCB is used to transfera command list as data IUs and the other TCCB (which is sent to thecontrol unit in a different transport command (TC_IU)) is used totransfer customer data as data IUs.

One example of an I/O processing system incorporating and using one ormore aspects of the present invention is described with reference toFIG. 1. I/O processing system 100 includes a host system 101, whichfurther includes for instance, a main memory 102, one or more centralprocessing units (CPUs) 104, a storage control element 106, and achannel subsystem 108. The host system 101 may be a large scalecomputing system, such as a mainframe or server. The I/O processingsystem 100 also includes one or more control units 110 and one or moreI/O devices 112, each of which is described below.

Main memory 102 stores data and programs, which can be input from I/Odevices 112. For example, the main memory 102 may include one or moreoperating systems (OSs) 103 that are executed by one or more of the CPUs104. For example, one CPU 104 can execute a Linux® operating system 103and a z/OS® operating system 103 as different virtual machine instances.The main memory 102 is directly addressable and provides for high-speedprocessing of data by the CPUs 104 and the channel subsystem 108.

CPU 104 is the controlling center of the I/O processing system 100. Itcontains sequencing and processing facilities for instruction execution,interruption action, timing functions, initial program loading, andother machine-related functions. CPU 104 is coupled to the storagecontrol element 106 via a connection 114, such as a bidirectional orunidirectional bus.

Storage control element 106 is coupled to the main memory 102 via aconnection 116, such as a bus; to CPUs 104 via connection 114; and tochannel subsystem 108 via a connection 118. Storage control element 106controls, for example, queuing and execution of requests made by CPU 104and channel subsystem 108.

In an exemplary embodiment, channel subsystem 108 provides acommunication interface between host system 101 and control units 110.Channel subsystem 108 is coupled to storage control element 106, asdescribed above, and to each of the control units 110 via a connection120, such as a serial link. Connection 120 may be implemented as anoptical link, employing single-mode or multi-mode waveguides in a FibreChannel fabric (e.g., a fibre channel network). Channel subsystem 108directs the flow of information between I/O devices 112 and main memory102. It relieves the CPUs 104 of the task of communicating directly withthe I/O devices 112 and permits data processing to proceed concurrentlywith I/O processing. The channel subsystem 108 uses one or more channelpaths 122 as the communication links in managing the flow of informationto or from I/O devices 112. As a part of the I/O processing, channelsubsystem 108 also performs the path-management functions of testing forchannel path availability, selecting an available channel path 122 andinitiating execution of the operation with the I/O devices 112.

Each channel path 122 includes a channel 124 (channels 124 are locatedwithin the channel subsystem 108, in one example, as shown in FIG. 1),one or more control units 110 and one or more connections 120. Inanother example, it is also possible to have one or more dynamicswitches (not depicted) as part of the channel path 122. A dynamicswitch is coupled to a channel 124 and a control unit 110 and providesthe capability of physically interconnecting any two links that areattached to the switch. In another example, it is also possible to havemultiple systems, and therefore multiple channel subsystems (notdepicted) attached to control unit 110.

Also located within channel subsystem 108 are subchannels (not shown).One subchannel is provided for and dedicated to each I/O device 112accessible to a program through the channel subsystem 108. A subchannel(e.g., a data structure, such as a table) provides the logicalappearance of a device to the program. Each subchannel providesinformation concerning the associated I/O device 112 and its attachmentto channel subsystem 108. The subchannel also provides informationconcerning I/O operations and other functions involving the associatedI/O device 112. The subchannel is the means by which channel subsystem108 provides information about associated I/O devices 112 to CPUs 104,which obtain this information by executing I/O instructions.

Channel subsystem 108 is coupled to one or more control units 110. Eachcontrol unit 110 provides logic to operate and control one or more I/Odevices 112 and adapts, through the use of common facilities, thecharacteristics of each I/O device 112 to the link interface provided bythe channel 124. The common facilities provide for the execution of I/Ooperations, indications concerning the status of the I/O device 112 andcontrol unit 110, control of the timing of data transfers over thechannel path 122 and certain levels of I/O device 112 control.

Each control unit 110 is attached via a connection 126 (e.g., a bus) toone or more I/O devices 112. I/O devices 112 receive information orstore information in main memory 102 and/or other memory. Examples ofI/O devices 112 include card readers and punches, magnetic tape units,direct access storage devices, displays, keyboards, printers, pointingdevices, teleprocessing devices, communication controllers and sensorbased equipment, to name a few.

One or more of the above components of the I/O processing system 100 arefurther described in “IBM® z/Architecture Principles of Operation,”Publication No. SA22-7832-05, 6th Edition, April 2007; U.S. Pat. No.5,461,721 entitled “System For Transferring Data Between I/O Devices AndMain Or Expanded Storage Under Dynamic Control Of Independent IndirectAddress Words (IDAWS),” Cormier et al., issued Oct. 24, 1995; and U.S.Pat. No. 5,526,484 entitled “Method And System For Pipelining TheProcessing Of Channel Command Words,” Casper et al., issued Jun. 11,1996, each of which is hereby incorporated herein by reference in itsentirety. IBM is a registered trademark of International BusinessMachines Corporation, Armonk, N.Y., USA. Other names used herein may beregistered trademarks, trademarks or product names of InternationalBusiness Machines Corporation or other companies.

Turning now to FIG. 2, one embodiment of the control unit 110 and thechannel 124 of FIG. 1 that support chain-paired TCW channel programexecution is depicted in greater detail. The control unit 110 includesCU control logic 202 to parse and process command messages containingone or more TCCBs, received from the channel 124 via the connection 120.The CU control logic 202 can extract DCWs and control data from theTCCB(s) received at the control unit 110 to control a device, forinstance, I/O device 112 via connection 126. The CU control logic 202sends device commands and data to the I/O device 112, as well asreceives status information and other feedback from the I/O device 112.The CU control logic 202 may use CU chain-pair logic 204 to performvarious checks of the command messages received at the control unit 110,as well as determine an appropriate response. For example, the CUchain-pair logic 204 can inform the channel 124 of the maximum number oflinked commands that are supported. The CU chain-pair logic 204 may alsohandle padding, incorrect length suppression, chain linking at the DCWlevel, and command/data allocation per message. While the CU chain-pairlogic 204 is depicted separately from the CU control logic 202, it willbe understood that the CU chain-pair logic 204 can be incorporated aspart of the CU control logic 202.

The CU control logic 202 can access and control other elements withinthe control unit 110, such as CU timers 206 and CU registers 208. The CUtimers 206 may include multiple timer functions to track how much time asequence of I/O operations or a single I/O operation takes to complete.The CU timers 206 may further include one or more countdown timers tomonitor and abort I/O operations and commands that do not completewithin a predetermined period. In an exemplary embodiment, the CU timers206 continue to run between chain-paired TCCBs until the chain completesas an I/O operation spanning multiple TCCBs. The CU registers 208 caninclude fixed values that provide configuration and status information,as well as dynamic status information that is updated as commands areexecuted by the CU control logic 202. The control unit 110 may furtherinclude other buffer or memory elements (not depicted) to store multiplemessages or status information associated with communications betweenthe channel 124 and the I/O device 112. The CU registers 208 may includea maximum linked commands parameter that defines the maximum number ofstreamed command messages for one I/O operation that the control unit110 supports.

The channel 124 in the channel subsystem 108 includes multiple elementsto support communication with the control unit 110. For example, thechannel 124 may include CHN control logic 210 that interfaces with CHNsubsystem timers 212 and CHN subsystem registers 214. In an exemplaryembodiment, the CHN control logic 210 controls communication between thechannel subsystem 108 and the control unit 110. The CHN control logic210 may directly interface to the CU control logic 202 via theconnection 120 to send commands and receive responses, such as transportcommand and response information units (IUs). Alternatively, messaginginterfaces and/or buffers (not depicted) can be placed between the CHNcontrol logic 210 and the CU control logic 202. The CHN subsystem timers212 may include multiple timer functions to track how much time asequence of I/O operations takes to complete, in addition to the timetracked by the control unit 110. The CHN subsystem timers 212 mayfurther include one or more countdown timers to monitor and abortcommand sequences that do not complete within a predetermined period.The CHN subsystem registers 214 can include fixed values that provideconfiguration and status information, as well as dynamic statusinformation, updated as commands are transported and responses arereceived.

In an exemplary embodiment, the channel subsystem 108 further includesCHN chain-pair logic 216. The CHN chain-pair logic 216 can manage chainlinking and allocation of commands and data to separate messages for thechannel 124. Although the CHN chain-pair logic 216 is depictedseparately from the CHN control logic 210, it will be understood thatthe CHN chain-pair logic 216 can be incorporated as part of the CHNcontrol logic 210.

FIG. 3 depicts an embodiment of a chained-pair TCW channel program 300that includes TCW 302 chain linked to TCW 304. The TCW 302 includeslinks to TCCB 306, a DCW list 308 as data, and a transport status block(TSB) 310. The TCW 304 includes links to a TCCB 312, TSB 310, and twodata areas (read data area 314 and write data area 316). The TCCB 306includes a DCW indicating that data associated with the TCCB is actuallya list of commands (DCW list 308), not customer data. The TCCB 312includes a DCW indicating that it is a companion to TCCB 306, and assuch, the read data area 314 and write data area 316 are for reading andwriting data associated with commands in the DCW list 308. Thus, thepair of TCWs 302 and 304 provides links and information to establish thepair relationship between TCCB 306 and 312 to perform a complete I/Ooperation comprised of multiple commands and data. The various links toTCCBs and data areas, such as TCCBs 306 and 312, can be direct orindirect references to areas of memory. For example, transport blocksand data areas 306, 308, 312, 314 and 316 can be further subdivided intosmaller blocks (contiguous or non-contiguous) and managed using indirectlists pointing to the smaller blocks (e.g., lists of transport modeindirect data address words (TIDALs)). In an exemplary embodiment, theTCCBs 306 and 312 are sent from channel subsystem 108 of FIG. 1 to atargeted control unit 110 that parses and executes commands in the TCCBs306 and 312. As part of parsing and executing the commands in the TCCBs306 and 312, the control unit 110 determines that chain-pairing isemployed, with the actual device level commands held in data IUs, andprocesses the commands in the DCW list 308 accordingly. The TCCB 306 mayappear as a write command with associated data, but the data can be acombination of read and write DCWs. The TSB 310 may remain at thechannel subsystem 108 to hold status information associated with theexecution of commands associated with TCCBs 306 and 312 at the controlunit 110, enabling OSs 103 to access status information. The data areas314 and 316 can be used to hold write data to send to the control unit110 or read data received from the control unit 110 as driven bycommands in the DCW list 308.

In an exemplary embodiment, the chained-pair TCW channel program 300represents a large channel program chained-paired across the TCWs 302and 304 and TCCBs 306 and 312. For example, the chained-pair TCW channelprogram 300 can send 1000 or more DCWs in the DCW list 308 using onlytwo TCCBs. The TCWs 302 and 304 each include a TSB address pointing tothe same TSB 310. If the channel program ends successfully, only the TSBaddress in the last TCW 306 is used by the channel 124; however, if thechannel program ends early for whatever reason, the channel 124 can usesthe TSB address in any TCW that the channel 124 may be working with, toobtain the memory address to store ending status in the TSB 310.

It will be understood that the configuration of and number of TCWs 302and 304 merely represents an embodiment, and is not limiting in scope,as there could be any number of TCWs chain-pair linked (e.g., multiplechain-pairs chained together or chain linked TCWs chained together withchain-paired TCWs) as part of the channel program 300.

In order to determine whether a control unit can support chain linkedTCW channel programs, a compatibility link protocol may be employedprior to sending chain linked TCCBs to the control unit. An example of acompatibility link protocol is depicted in FIG. 4. Channel 400 sends aprocess login (PRLI) request 404 to the control unit 402 in a defaultcommunication format. The control unit 402 responds with a PRLI accept406, which may include information defining communication parametersthat are acceptable to the control unit 402. In response to receivingthe PRLI accept 406, the channel may proceed with sending chain linkedTCCBs to the control unit 402 for execution, such as chain-pair linkedTCCBs 306 and 312. Other messages may also be exchanged between thechannel 400 and the control unit 402 as part of link initialization andconfiguration. The channel 400 and the control unit 402 representembodiments of the channel 124 and control unit 110 of FIG. 1.

FIG. 5 depicts an example of a PRLI Request message 500, whichrepresents an embodiment of the PRLI request 404 of FIG. 4. The payloadof the PRLI Request message 500 may include a service parameter page,which includes service parameters for one or all image pairs.

The service parameter page of the PRLI Request message 500 may includemultiple fields, such as type code 502, type extension 504, maximuminitiation delay time 506, flags 508, and max linked commands 510. Eachfield in the page of the PRLI Request message 500 is assigned to aparticular byte address. Although one arrangement of fields within thepage of the PRLI Request message 500 is depicted in FIG. 5, it will beunderstood that the order of fields can be rearranged to alternateordering within the scope of the disclosure. Moreover, fields in thepage of the PRLI Request message 500 can be omitted or combined withinthe scope of the invention.

The type code field 502, located at word 0, byte 0, represents theprotocol type code, such as the Fibre Channel Single Byte Protocol typecode. For example, a value of “1B” hexadecimal in this byte indicatesthat this service parameter page 500 is defined in the selected protocol(e.g., Fiber Channel single byte). The type extension 504, located atword 0, byte 1, may further supplement the type code field 502.

The maximum initiation delay time field 506, located at word 3, byte 0,provides the maximum time (e.g., in seconds) that the channel 124 ofFIG. 1 can allow in the Initiation Delay Time field in a process Logout(PRLO) from the control unit 110.

Flags 508, in an exemplary embodiment, has the following definition:

Bit 0—Transport Mode/Command Mode. A value of this bit set to one (1)means that the sender supports both Command Mode and Transport Mode. Ifthe bit is set to zero (0), the sender only supports Command Mode. Ifthe channel 124 sets this bit to a one, then the control unit 110 mayrespond with this bit set to one if it supports Transport Mode.

Bits 1-6—Reserved.

Bit 7—First Transfer Ready for Data Disabled. If both the channel 124and control unit 110 choose to disable the first write transfer readyinformation unit (XFER_RDY IU), then the first TC_IU of all I/Ooperations performing writes between the channel 124 and control unit110 operate without using the XFER_RDY IU before the first datainformation unit (Data IU) is transmitted for the first TC_IU of an I/Ooperation. The XFER_RDY IU is transmitted to request each additionalData IU, if any for the current TC_IU and all data IUs for any followingTC_IUs for the channel program if any.

The max linked commands field 510 indicates the maximum count ofadditional Transport Command information units (TC_IUs) that the channel124 supports for streaming to the control unit 110 as chain linkedcommands for the same I/O device 112 after the first TC_IU has been sentto the control unit 110. Values may range from 0 to 15, with a value ofzero meaning that the channel 124 does not support chain linking ofTC_IUs. A value of X equal to one to fifteen indicates that the channel124 will send out X TC_IUs after the first TC_IU for the same I/O device112 (if there are X TCWs chain linked together) and then send out onenew TC_IU for each previous TC_IU that completed until the channelprogram is completely executed. In an exemplary embodiment, eachchained-pair of TC_IUs counts as a single chain linked command.

In one exemplary embodiment, the remaining fields in the page of thePRLI Request message 500 are reserved and/or set to zero (0). Forexample, bytes 2 and 3 of word 0, and words 1 and 2 are set to zero.Byte 1 and a portion of byte 2 of word 3 may also be reserved.

Turning now to FIG. 6, an example of a PRLI Accept message 600 isdepicted, which represents an embodiment of the PRLI accept 406 of FIG.4. The payload of the PRLI Accept message 600 may include a serviceparameter page. The service parameter page of the PRLI Accept message600 may include multiple fields, such as type code 602, type extension604, response code 606, first burst size 608, flags 610, and max linkedcommands 612. Each field in the page of the PRLI Accept message 600 isassigned to a particular byte address. Although one arrangement offields within the page of the PRLI Accept message 600 is depicted inFIG. 6, it will be understood that the order of fields can be rearrangedto alternate ordering, or can be omitted or combined, within the scopeof the disclosure.

The type code field 602, located at word 0, byte 0, is the protocol typecode, and is similar to the type code field 502 of FIG. 5. The typeextension field 604, located at word 0, byte 1, corresponds to the typeextension field 504 of FIG. 5.

The response code field 606, located at word 0, byte 2, bits 4-7, isdefined by its corresponding protocol, such as the Fibre Channel Framingand Signaling protocol (FC-FS), which is described further in “ANSIINCITS 433-2007, Information Technology Fibre Channel Link Services(FC-LS)”, July 2007, which is hereby incorporated herein by reference inits entirety.

The First Burst Size field 608, located at word 3, bytes 0-1, bits 0-15,provides the maximum amount of data (e.g., the maximum number of 4 kbyte blocks of data) allowed in the first Data IU that is sentimmediately after the first TC_IU, when the First Transfer Ready forData Disabled flag bit (word 3, byte 3, bit 7) is set to one. A value ofzero in this field indicates that there is no specified first burstsize.

Flags 610 are similar to the flags 508 of FIG. 5 described inconjunction with the PRLI Request message 500. The control unit 110 setsvalues to these flags that correspond to the mode of operation it willrun with the channel 124.

In an exemplary embodiment, the max linked commands field 612 is themaximum count of streamed TC_IUs that the control unit 110 supports forone I/O operation. The control unit 110 responds with a count equal toor less than the value the channel 124 sent to the control unit 110 inthe service parameter page for the PRLI Request message 500. The channel124 uses the count received from the control unit 110 as the maximumnumber of linked TC_IUs queued at the control unit 110. If the controlunit responds with a count of zero, means the control unit 110 does notsupport chain linking of TC_IUs.

In one exemplary embodiment, the remaining fields in the page of thePRLI Accept message 600 are reserved and/or set to zero (0). Forexample, bits 1-3 of word 0, byte 2, and words 1 and 2 are set to zero.Byte 3 of word 0 is reserved and set to zero. A portion of byte 2 ofword 3 may also be reserved.

Once the channel 124 and the control unit 110 establish that TransportMode is supported and the maximum number of linked commands isestablished, a TCW channel program with chained-pair linking can beexecuted. FIG. 7 depicts one embodiment of a link protocol used tocommunicate between a channel 700 and control unit 702 to execute thechained-pair TCW channel program of FIG. 3, where the channel 700 andcontrol unit 702 are embodiments of the channel 124 and control unit 110of FIG. 1. An OS, such as OS 103 of FIG. 1, builds the TCWs 302 and 304associated control blocks TCCB 306, DCW list 308 and TCCB 312 shown inFIG. 3 and executes a start subchannel command with an address in anoperation request block that points to TCW 302. This results in thechannel 700 fetching TCW 302 and the associated TCCB 306.

The channel 700 sends TCCB 306 in TC_IU 704, opening exchange A with asequence number of one to the control unit 702. In this example, a chainlinked flag bit is set to one in the TCW 302, indicating that the TCW302 is chain linked to another TCW (TCW 304) as defined by a next TCWaddress field in the TCW 302. The TCW 302 also includes an asserted nextTCW/TCCB companion flag bit to inform channel 124 that the next TCCB(TCCB 312) is a companion to the present TCCB 306 resulting inpreventing the sequence number and counts associated with the max linkedcommands from incrementing for the chained-pair. Therefore, because thechain linked flag bit is set to one and the next TCW/TCCB companion flagbit is set to one in the TCW 302, the channel 700 fetches TCW 304 andassociated TCCB 312, sending TCCB 312 in TC_IU 706, with a sequencenumber of one in exchange B to the control unit 702. Here, the sequencenumber is the same for TC_IU 704 and 706 because they are part of thesame pair; otherwise, the sequence number would increment between eachTC_IU in the chain.

Although the two TC_IUs 704 and 706 are sent one right after the otheron a fibre channel link, each as a separate exchange to the control unit702, the TC_IUs 704 and 706 may arrive at the control unit 702 in adifferent order than they were sent, as communication can be inparallel. The control unit 702 analyzes the sequence numbers, flags, andDCWs in the TC_IUs 704 and 706 that are received to determine the orderof execution and further processing to perform prior to execution. Inthe example of FIG. 7, since TC_IUs 704 and 706 have the same sequencenumber and are companions in the same chained-pair, the control unit 702further analyzes DCWs in the TC_IUs 704 and 706 to determine which TC_IU704 or 706 has commands in its associated data IUs versus customer data.A chain linked flag bit set to zero in the TCW 304 informs the channel700 that the TCCB 312 is the final TCCB in the channel program 300 tosend to the control unit 702.

The control unit 702 executes the two TC_IUs 704 and 706 as a pair,starting with TC_IU 704, which includes a last DCW command a (00h) DCWcommand with a data DCW flag bit set to a zero. The control unit 702sends a Transfer Ready IU 708 on exchange A, to request DCWs transferredas Data IUs 710 for the (00h) DCW command in TC_IU 704. The channel 700sends the data (DCW list 308) requested by the control unit 702 forTC_IU 704 to the control unit 702 as Data IUs 710 on exchange A. Thecontrol unit 702 starts executing the DCWs received as data for TC_IU704, using exchange B and data byte counts from TC_IU 706.

Assuming the first set of DCWs received for TC_IU 704 are read DCWs, thecontrol unit 702 sends customer data read from a targeted I/O device(e.g., I/O device 112 of FIG. 1) as Data IUs 712 to the channel onexchange B. The control unit 702 sends a Transfer Ready IU 714 onexchange A, to complete the request for the rest of the DCWs that aretransferred as Data IUs 716 for the (00h) DCW command in TC_IU 704. Thechannel 700 sends the rest of the data (DCW list 308) requested by thecontrol unit 702 for TC_IU 704 to the control unit 702 as Data IUs 716on exchange A.

Assuming that the second set of DCWs received for TC_IU 704 are writeDCWs, the control unit 702 sends a Transfer Ready IU 718 to request thecustomer data from the channel 700 on exchange B. The control unit 702sends a Transport Response (Status) IU 720 with zero FCP status to closeexchange A and to inform the channel 700 that TC_IU 704 has completed,in that all the data (DCWs) have been fetched for TC_IU 704; however,completion does not mean that the DCWs have been executed. The state ofexecution of the DCWs requested from TC_IU 704 is reported in theTransport Response (Status) IU 724 for TC_IU 706. In an exemplaryembodiment, the Transport Response IU 720 is a basic status message thatconveys minimal information, e.g., a 24-byte status message. This iscontrasted against a more elaborate Transport Response IU 724 withextended status that provides more comprehensive information and may bemuch greater in length, e.g., 48 to 64 (or more) bytes, at theconclusion of an I/O operation.

The channel 700 sends the customer data requested by the control unitfor TC_IU 706 to the control unit 702 as Data IUs 722 on exchange B.When the control unit 702 completes TC_IU 706, it sends the completeTransport Response IU 724 (including extended status), which closesexchange B and informs the channel 700 that TC_IU 706 (and the entireI/O operation depicted in FIG. 7) has completed. The final status may bewritten to the TSB 310 of FIG. 3. The channel 700 can present the finalstatus to the OS (e.g., OS 103 of FIG. 1), informing the OS that the I/Ooperation is now completed.

In an exemplary embodiment, extended status includes various timingparameters that can be continued between TC_IUs, such as TC_IUs 704 and706, as calculated using CU timers 206 of FIG. 2. For example, extendedstatus can include a total device time parameter, defer time parameter,queue time parameter, device busy time parameter, device active onlytime parameter, and appended device sense data. The total device timeparameter is the elapsed time from when the control unit 702 receivedthe TC_IU 704 until sending the transport response IU 724 for the I/Ooperation. The defer time parameter indicates control unit defer time.This is the time accumulated by the control unit 702 working with theI/O device (e.g., I/O device 112) when no communication with the channel700 is performed. The queue time parameter is the time that an I/Ooperation is queued at the control unit 702, but does not include queuetime for device busy time where the I/O device is reserved by anotherchannel 700 under control of a different OS (e.g., OS 103) on the samesystem or on another system. The device busy time parameter is the timethat a TC_IU is queued at the control unit 702 waiting on a device busycaused by the I/O device being reserved by another channel 700 undercontrol of a different OS on the same system or on another system. Thedevice active only time parameter is the elapsed time between a CE and aDE at the control unit 702, when the control unit 702 holds the CE untilDE is available. The appended device sense data is supplemental statusthat the control unit 702 provides conditionally in response to anactive unit check (UC) bit in the device status.

An exemplary embodiment of a transport control word (TCW) 800 isdepicted in FIG. 8. The TCW 800 may be utilized by the channel 124 ofFIG. 1 to set up the I/O operation and is not sent to the control unit110. The TCW depicted in FIG. 8 provides for both input and output datawithin a single I/O operation. The TCW 800 illustrates formatting thatcan be used for a variety of TCWs, such as TCWs 302 and 304 of FIG. 3.

In the exemplary TCW 800 depicted in FIG. 8, a format field 802 equal to“00” binary indicates that what follows is a standard TCW 800, withother values (e.g., 01, 10, 11) equating to TCW format variations. TheTCW 800 may include reserved bits 804 for possible future use.

The TCW 800 also includes a flags field 806. Reserved flags in the flagsfield 806 may be set to zero. Examples of flags bits that are mapped tothe flags field 806 include a chain linked flag bit, a next TCW/TCCBcompanion bit, a TIDAL read flag, a TCCB TIDAL flag, and a TIDAL writeflag.

When the chain linked flag bit set to a one, this informs the channel124 that the next TCW address field 828 is to be used as the next TCW tobe executed for the continuation of the I/O program. Counters, timers,and status tracking (e.g., CU timers 206 and/or CHN subsystem timers 212of FIG. 2) can continue from one TCCB to the next TCCB when the chainlinked flag is set to a one, such as between TCCBs 306 and 312.Exchanges may be closed by the control unit 110 for the intermediateTCCBs that were executed successfully with an equivalent of FCP zerostatus in the associated transport response IU, as described in theexample of FIG. 7. A full transport response IU with extended status isnot transferred until the last TCCB of the chain linked channel programis executed or until the control unit 110 encounters an early endcondition. Since the TCW 800 remains local to the channel 124, the stateof the chain linked flag can be sent to the control unit 110 as a chainlinked TCCB flag in a TCCB as part of a TC_IU.

The next TCW/TCCB companion flag bit informs the channel 124 to send thenext TCCB to the control unit 110 with the same sequence number, and tocount the two TC_IUs as one IU for max linked commands accounting set upwith the process login of FIGS. 4-6. This establishes a chained-pair ofcommands.

In an exemplary embodiment, the TIDAL read flag is set to one wheninput-data address field 818 contains an address of a TIDAL. If theTIDAL read flag is set to zero, then the input-data address field 818contains a data address. In an exemplary embodiment, the TCCB TIDAL flagis set to one when TCCB address field 822 contains an address of aTIDAL. If the TCCB TIDAL flag is set to zero, then the TCCB addressfield 822 directly addresses the TCCB. The TCCB TIDAL flag allows theoperating system software or hyper-visor to layer function and prefixuser channel programs. In an exemplary embodiment, the TIDAL write flagis set to one when output-data address field 816 contains an address ofa TIDAL. If the TIDAL write flag is set to zero, then the output-dataaddress field 816 contains a data address.

The TCW 800 also includes a TCCB length field 810 which indirectlyrepresents the length of the TCCB and may be utilized to determine theactual length of the TCCB.

Read/write bits 812 in the TCW 800 are utilized to indicate whether datais being read and/or written as a result of executing the TCW 800. In anexemplary embodiment, the read bit in the read/write 812 bits is set toone to indicate that input data is being transferred from an I/O device112 to system storage (e.g., main memory 102) in the host system 101 asa result of executing the TCW 800. The write bit in the read/write bits812 is set to one to indicate that output data is being transferred fromsystem storage (e.g., main memory 102) in the host system 101 to an I/Odevice as a result of executing the TCW 800. The TCW 304 of FIG. 3 is anexample of a TCW with reads and writes.

The output-data address field 816 includes the address for the outputdata (if any). As described previously, the contents of the output-dataaddress field 816 may be an address of a TIDAL for output data (e.g., anindirect address) or the actual address of the output data (e.g., adirect address). The input-data address field 818 includes the addressfor the input data (if any). As described previously, the contents ofthe input-data address field 818 may be an address of a TIDAL for inputdata or the actual address of the input data.

The TCW 800 also includes a transport-status-block address field 820. Aportion (e.g., the extended status part) of a completion status in atransport response IU for an I/O operation is stored at this address.The TCCB address field 822 in the TCW 800 includes an address where theTCCB is located in system storage. As described previously, the TCCB isthe control block where the DCWs to be executed for the TCW 800 reside.Also as described previously, the contents of the TCCB address field 822may be an address of a TIDAL for the TCCB or the actual address of theTCCB.

The output count field 824 in the TCW 800 indicates the amount of outputdata to be transferred by the TCW/TCCB for an output operation. In anexemplary embodiment, the output count field 824 specifies the number ofbytes in the output storage area designed by the TCW (the output-dataaddress 816) to be transferred. The input count field 826 in the TCW 800indicates the amount of input data to be transferred by the TCW/TCCB foran input operation. In an exemplary embodiment, the input count field826 specifies the number of bytes in the input storage area designed bythe TCW (the input-data address 818) to be transferred.

The next TCW address field 828 is added to the TCW 800 that has thechain linked flag bit set in the flags field 806. The next TCW addressfield 828 is used to point to the address of the next TCW to be executedas part of a chain, such as that depicted in FIG. 3.

The interrogate-TCW address field 830 contains the address of anotherTCW and is used by the channel 124 to interrogate that state of anoperation under the initiative of a cancel sub-channel I/O instruction.This field may be assigned for the first TCW in a chain, and used forother purposes for subsequent chained TCWs.

The TCW 800 depicted in FIG. 8 is one example of how a TCW can beconfigured. Other configurations are possible where additional fieldsare included and/or fields depicted in FIG. 8 are not included.

One example of a command message 900, e.g., a transport command IU,communicated from the channel subsystem 108 to the control unit 110 toexecute a TCW channel program is depicted in FIG. 9. The command message900 illustrates formatting that can be used for a variety of TC_IUs,such as TC_IUs 704 and 706 of FIG. 7. The command message 900 includes aheader 902, a transport command header (TCH) 904, a transport commandarea header (TCAH) 906, a transport command area (TCA) 908, and atransport command area trailer (TCAT) 910. In an exemplary embodiment,the TCCBs 306 and 312 of FIG. 3 utilize formatting as depicted in theTCAH 906, TCA 908, and TCAT 910.

The header 902 may include multiple words as address header 912,defining the highest level of header in the command message 900. Theheader 902 may include information such as channel and control unitimage IDs and a device address.

The TCH 904 includes a sequence number 913. The sequence number 913informs the control unit 110 of the order to execute multiple commandmessages 900 (e.g., TC_IUs 704 and 706 of FIG. 7) that are all part ofthe same channel I/O operation targeting an I/O device (e.g., I/O device112). The sequence number 913 starts at (01h) in the first TC_IU foreach start to the I/O device 112 independent of the value it ended onfor the last start to the same I/O device 112 and increments by one foreach subsequent TC_IUs except for chained-pair linked TC_IUs. Forchain-pair linked TC_IUs both TC_IUs of the chained-pair have the samesequence number 913. If an I/O operation only contains one TCW/TCCB,then the value of the sequence number 913 is set to zero. The TC_IUschain linked together are executed in the order of the sequence numbers,even if the TC_IUs are received at the control unit 110 out of order.

The TCH 904 includes task information 914, which may be set to areserved value, e.g., zero, while operating in transport mode. The TCH904 also includes L1 length 916 and read/write field 918. The L1 length916 defines the length of the TCA 908 in words+1. The L1 length 916 canbe used to limit and define the size of the TCA 908. The read/writefield 918 defines whether read data, write data, or no data is beingtransferred in the command message 900, where a read is a transfer fromthe control unit 110 to the channel subsystem 108.

The TCAH 906 includes format field 920 and control field 922. The formatfield 920 and control field 922 may be set to fixed values, such as 7Fhexadecimal and zero respectively, to indicate that a variable lengthformat is used, as defined by SPC-4. SPC-4 is further described in “SCSIPrimary Commands-4 (SPC-4)”, Project T10/1731-D, Rev 11, INCITS (May2007), which is hereby incorporated herein by reference in its entirety.The TCAH 906 additionally includes reserved fields 924 and 926,TCCB-flags 927, as well as L2 length 928.

The TCCB-flags 927 inform the control unit 110 about the characteristicsof the command message 900 (the current TC_IU). The TCCB-flags 927 mayinclude a chain linked TCCB flag bit. The chain linked TCCB flag set toa one informs the control unit 110 that there is another TC_IU followingthe current TC_IU that is part of the same I/O operation. Counters,timers, and status tracking (e.g., CU timers 206 and/or CHN subsystemtimers 212 of FIG. 2) can continue from one TCCB to the next TCCB whenthe chain linked TCCB flag is set to a one, and a CC bit is set to a onein the last DCW (e.g., DCW 946) in the TCA 908 for this TC_IU. Theexchange may be closed when the TC_IU is executed successfully with anequivalent of FCP zero status, which equates to channel end (CE), deviceend (DE) only status. The CE may indicate that a portion of the I/Ooperation involving a transfer of data or control information betweenthe channel 124 and the control unit 110 has been completed. The DE mayindicate that a device portion of an I/O operation is completed. Noextended status is transferred until the last TC_IU for the TCW channelprogram is executed or for the TC_IU that ended the TCW channel program.The channel 124 sends the next TC_IU to the control unit 110 based onthe value of a TC_IU streaming count, which can be tracked in the CHNsubsystem registers 214 of FIG. 2. The channel 124 sends TC_IUs up tothe max linked commands (e.g., max linked commands 612), and then sendthe subsequent TC_IUs as each previous TC_IU is completed.

The L2 length 928 is also referred to as transport-command-area length(TCAL), and may represent the number of bytes after this position in thecommand message 900. The L2 length 928 limits the size of the TCA 908.The TCAH 906 further includes a service action code 930, reserved field932, priority 934, and reserved field 936. The service action code 930defines the type of DCWs used in the TCA 908. The priority 934 can beset equivalent to a priority byte of a FICON command header as definedin FC-SB-3.

The TCA 908 includes DCW one and control data 940, DCW two 942, DCWthree 944, and DCW four 946. The DCW one and control data 940 includesDCW fields such as a command 948, flags field 950, a reserved field 952,control data (CD) count 954, and data byte count 956. The command 948may be equivalent to a CCW command byte, but directly interpreted by thecontrol unit 110 rather than the channel subsystem 108. The flags field950 includes reserved bits as well as one or more bits assigned toparticular functions, such as indicating whether an additional DCWexists in the TCA 908 as part of a command chain. The flags field 950may also include a pad flag bit, command chain (CC) flag bit, a suppressincorrect length (SLI) flag bit, and a data DCW flag bit.

The pad flag bit adds padding on read short counts. If the pad and SLIflag bits are set to one and the command 948 is a read, the associateddata (e.g., data IUs 712 associated with TC_IU 706) are filled withzeros to the end of the data byte count 956 if the record is shorterthan the data byte count 956. In an exemplary embodiment, padding on aread is not performed if the SLI flag bit is not set to a one. The padflag bit is a “don't care” for write DCWs.

Software can make the count fields in the DCWs consistent with TIDALcounts fields. The data stream between the channel 124 and control unit110 is aligned to the DCW counts in the TCA 908 even if the record isshorter than the count field. To allow the TCW channel program tocontinue when the record is short and software set the SLI flag bit, thepad flag bit is set to allow the control unit 110 to pad out the datastream with zeros until the count field in the DCW is satisfied (e.g.,data byte count 956). For example, if a read command requests 4kilobytes of data, but the record on the I/O device 112 is only 2kilobytes in length, the control unit 110 can insert additional zeros topad the returned data IU to 4 kilobytes of data. The exception to thisis for the last DCW in the TCA 908 (e.g., DCW 946) or the last DCWexecuted in the TCA 908 should execution terminate early.

If for any reason the control unit 110 cannot satisfy the DCW byte count956, in terms of the bytes of data sent to, or received from the channel124 on a DCW that is not the last DCW of the TCA 908, the control unit110 ends at a DCW with an incorrect length flag bit set in a statusflags byte of the transport response IU.

The CC flag bit indicates a command chain to the next DCW in the TCA908. The CC flag bit set to zero means that the associated DCW is thelast DCW of the program. The CC flag bit can be set in the last DCW ofthe TCA 908 if the chain linked TCCB flag is set in the TCCB-flags field927 and the chain linked flag bit is set in the flags field 806 in theTCW 800.

The SLI flag bit may be used for long record writes or reads, where along record is larger than the DCW count (e.g., greater than data bytecount 956), and short records on write and short record on reads if thepad flag bit is set to a one.

When the SLI flag bit is set to a one, the data byte count 956 of datais transferred on a write, even if the data is not used by the controlunit 110. If the SLI flag bit is not set to a one, then the chain isended if an early end condition is encountered. However, data may berequested up to the next cyclic redundancy check (CRC) checking boundaryin order to check the CRC on the data already received before the datacan be committed to media.

The SLI flag bit may be set for short record reads if the pad flag bitis set to a one or it is the last DCW in TCA 908. The SLI flag bit doesnot suppress incorrect length for short records on a read DCW unless theDCW is the last DCW in a DCW list or the pad flag bit is set to a one.When the pad flag bit is also set to a one, the control unit 110 padsout the record with zeros until the DCW count goes to zero. This keepsbyte counts in the TC_IU synchronized with TCW & TIDAL byte counts atthe channel 124.

The data DCW flag bit is used in conjunction with a DCW control command,which may be defined as a command 948 (or command 960/970/980) with avalue of zero. In an exemplary embodiment, the DCW control command isthe last DCW in the command message 900. No previous DCWs in the commandmessage 900 can transfer customer data. The previous DCW command(s) maybe DCW control commands with control data. The DCW control command isused by the control unit 110 to retrieve or receive a continuation ofthe DCW list 308 from the channel 124 for the I/O operation. The DCWcontrol command is not part of the DCW list 308, which provides commandsfor the I/O device 112. When the data DCW flag bit is zero, the databyte count 988 is the length of the DCW list 308. The Transport DataByte Count 992 is the length of the DCW list 308 plus CRC fields to bereceived from the channel 124 in data IUs. A CRC may be insertedperiodically, for instance, every 512 or 1024 bytes in the DCW list 308so that the control unit 110 can check the CRC before executing a DCW.The header 902 and sequence number 913 are the same between companionpairs that include a single DCW with a zero command and the data DCWflag bit is set to a one.

When the data DCW flag bit is a one, there may be only one DCW in thecommand message 900, (e.g., TC_IU 706) which is the data companion to aDCW control command in a TC_IU 704 with a data DCW flag bit of zero. Thedata byte count field 956 of this DCW is set to zero. The Transport databyte count fields 992 and 994 of this TC_IU 706 indicate the totalamount of read and write data to be transferred per count fields in theDCW list 308 from the channel 124 as requested by the companion DCWcontrol command in the companion TC_IU 704. The DCW list 308 has thesame format as the DCW list in the TCA 908. In the Transport Response IUfor the I/O operation, a DCW offset and DCW residual count are relativeto the DCW list 308 received in the Data IUs (e.g., Data IUs 710 & 716)using the companion TC_IU 704.

The CD count 954 is the byte count of control data 958. The CD count 954may be padded up to the next 4-byte boundary so that subsequent DCWsstart on a 4-byte boundary. The data byte count 956 is a four-byte countof data without padding, e.g., customer data. The control data 958exists when the CD count 954 is not zero. In the exemplary commandmessage 900, the DCW two 942, DCW three 944, and DCW four 946 containsubstantially similar fields as the DCW one and control data 940. Forexample, command 960, 970, and 980 are formatted in a similar fashion asthe command 948. Furthermore, flags field 962, 972, and 982 areformatted similar to the flags field 950. Additionally, CD count 966,976, and 986 are formatted similar the CD count 954, and data byte count968, 978, and 988 are similarly formatted to the data byte count 956.Although only four DCWs, including one DCW with control data (i.e., DCWone and control data 940) are depicted in the command message 900, itwill be understood that a varying number of DCWs with and withoutcontrol data can be included in the command message 900, including asingle DCW.

The TCAT 910 includes a longitudinal redundancy check (LRC) word 990calculated on the entire command message 900. The LRC word 990 can begenerated through applying an exclusive-or operation to an initial seedvalue with each field included in the LRC calculation in succession. TheTCAT 910 also includes a transport data byte count 992 indicating thetotal number of bytes transferred for a read or write I/O operation. Ifboth the read and write bits are active in read/write field 918, thenthe transport data byte count 992 is for the write data, andbidirectional read data length 994 in TCAT 910 is the read transportdata byte count.

Unusual ending conditions may be handled as follows when multipleTCWs/TCCBs are chained to form a chain-linked TCW channel program. Forchain linked TCWs channel programs, a halt subchannel command causes allactive exchanges to be aborted for the I/O device 112 and the subchannelto be returned to the OS 103 with primary, secondary and alert status. Aclear subchannel command for chain-linked TCW channel programs may causeall active exchanges to be aborted for the I/O device 112, followed bysending a selective reset command to the I/O device 112.

For the case where the channel 124 is sending multiple TCCBs (in TC_IUs)chain linked together to the control unit 110, if the control unit 110cannot execute any one of the TCCBs, the control unit 110 can sendterminating ending status, busy status (can only be sent in response tothe first TC_IU of a channel program) or retry status, with a statusconfirm, on the exchange for the TCCB that is ending early. The controlunit 110 also closes other outstanding exchanges for the same I/Ooperation, which have a sequence number greater than the sequence numberof the exchange on which the terminating status was sent. When thechannel 124 detects a terminating ending status IU with the request fora confirm request, the channel 124 stops sending new TCCBs to thecontrol unit 110 for that operation. All other exchanges for that I/Ooperation that are not closed after a timeout period (for example, 100milliseconds) are aborted by the channel 124. When all of the otherexchanges are closed for the I/O operation, the channel 124 sends theconfirm message, which closes the final exchange.

If one of the exchanges, out of many that were opened to send TCCBschain linked together to the control unit 110 is lost, the channel 124times out that exchange and send a Read Exchange Concise (REC) to thecontrol unit 110 inquiring about the exchange. If the control unit 110informs the channel 124 that it does not have the exchange, the channel124 aborts outstanding exchanges to the control unit 110 for the I/Ooperation.

FIG. 10 depicts a process 1000 for providing a chained-pair linked TCWchannel program at a channel subsystem in accordance with an exemplaryembodiment, and is described in reference to the I/O processing system100 of FIG. 1 and subsequent figures. The process 1000 is also describedin conjunction with process 1100 as depicted in FIG. 11 for providing achained-pair linked TCW channel program at a control unit, such asbetween channel 124 of channel subsystem 108 and control unit 110 ofFIG. 1. At block 1002, the channel 124 of channel subsystem 108configures a first command message specifying that DCW list 308 isencoded in a data message associated with the first command message aspart of the chained-pair linked transport control channel program. Atblock 1004, the channel 124 of channel subsystem 108 configures a secondcommand message chained-pair linked to the first command message, thesecond command message specifying data attributes (e.g., the amount ofcustomer data) associated with the DCW list 308. At block 1006, thechannel 124 of channel subsystem 108 transmits the first and secondcommand messages from the channel subsystem 108 to the control unit 110.At block 1008, the channel 124 of channel subsystem 108 transmits theDCW list 308 from the channel subsystem 108 to the control unit 110 inthe data message. The channel 124 of the channel subsystem 108 may waitfor a transfer ready message from the control unit 110 before sendingthe data message.

At block 1102, the control unit 110 receives the first command messagefrom channel 124 of the channel subsystem 108. The first command messagespecifies that DCW list 308 is encoded in a data message associated withthe first command message as part of the chained-pair linked transportcontrol channel program, such as TC_IU 704 with formatting as depictedin FIG. 9 in combination with Data IUs 710 and 716 of FIG. 7. The databyte count field 988 defines the size of the DCW list 308, which has theformat of the DCW list 308 as depicted in TCA 908. This list can beparsed into multiple data messages depending on the number of commandsand data message size constraints.

At block 1104, the control unit 110 receives the second command messagefrom channel 124 of the channel subsystem 108. The second commandmessage is chained-pair linked to the first command message andspecifies data attributes as the amount of customer data associated withthe DCW list 308, such as TC_IU 706 of FIG. 7 with formatting asdepicted in FIG. 9, where the transport data byte count fields 992 and994 defines the amount of read and write customer data for the DCW list308, such as Data IUs 712 (read) and 722 (write). To distinguish betweencommand messages that reference DCW lists versus customer data as partof a chained-pair, and to distinguish from non-chained-pair commands,multiple commands and/or flags can be used. For example, the controlunit 110 can use CU chain-pair logic 204 of FIG. 2 to locate and decodethe state of the data DCW flag bit and DCW control command in eachcommand message (e.g., TC_IU 704 and 706) to determine a course ofaction. In an exemplary embodiment, upon receiving the complete DCW list308, the control unit 110 transmits a transport response message withoutextended status.

At block 1106, the control unit 110 extracts the DCW list 308 from thedata message in response to receiving the data message. The control unit110 can read a chain linked flag (e.g., chain linked TCCB flag inTCCB-flags 927 of FIG. 9) in the first command message to determinewhether a subsequent command message is expected to follow the firstcommand message as part of the I/O operation, such as the second commandmessage. In response to determining that the subsequent command messageis expected, and upon executing the first set of one or more commandsreceived, the control unit 110 may continue to run counters associatedwith the I/O operation to span multiple command messages (e.g., CUtimers 206).

Various commands can be included in the DCW list 308, for instance readand write commands targeting I/O device 112. When the control unit 110executes a read command, it may determine that a record of dataassociated with the read command is smaller than a data count value(e.g., data byte count 956). The control unit 110 can check a pad flagassociated with the read command in the flags 950, 962, 972, and/or 982.The control unit 110 inserts pad data in a data message (e.g., one ormore of data IUs 708) in response to assertion of the pad flag (forexample, set to one). The pad data may be inserted up to the data countvalue, for instance, the value of data byte count 956, 968, 978, and/or988. This keeps communications and buffers aligned between the controlunit 110 and the channel 124. The control unit 110 may also compare therecord length relative to the data count value for read or writecommands, and check the SLI flag in the flags 950, 962, 972, and/or 982.The control unit 110 can suppress identification of an incorrect lengthcondition in response to assertion of the SLI flag when the recordlength does not match the data count value. The relationship between thepad and SLI flag bits may be as previously described.

The control unit 110 can also handle other error conditions. Forexample, the control unit 110 may determine that one or more commandsassociated with a communication exchange cannot execute. The controlunit 110 can respond sending a termination status message to the channel124 of the channel subsystem 108 indicating an inability to execute. Thecontrol unit 110 closes open communication exchanges with sequencenumbers greater than the sequence number associated with the one or morenon-executable commands. For example, if the control unit 110 hasreceived sequence numbers 1, 2, 3, and 4 on exchanges A, B, C, and D,and an error occurs in executing commands associated with sequencenumber 2, the control unit 110 can notify the channel 124 of the erroron exchange B with extended status, with a request for a confirm, onexchanges B and close exchanges C, and D. The channel 124 will closeexchange B with a confirm on exchange B after it has seen that exchangesA, C and D have been closed (assuming A closes after successfulcompletion of sequence 1 commands).

At block 1108, the control unit 110 executes the DCW list 308. Uponexecuting the device commands in the DCW list 308, control unit 110transmits a transport response message with extended status. Theextended status may include data and counter/timer values that spanacross the execution of TCCB 306, TCCB 312 and DCW list 308.

Technical effects of exemplary embodiments include chaining pairs ofTCWs and TCCBs together to form a transport control channel program thatis split and chained-pair linked between multiple TCWs and TCCBs for anI/O operation. Chained-pair linking allows a large number of commands tobe sent as one or more data messages according to a first transportcommand message, with customer data sent in data messages according to asecond transport command message. Rather than using transport commandmessages to directly send commands, a transport command message can beused to indicate that a large list of device command words are encodedas data associated with the command message. This enables transmissionof programs that would otherwise exceed existing message formattingconstraints, while reducing overhead related with transmitting the largeprogram. Additionally, support is provided for inserting padding andsuppressing incorrect length issues that may be associated with sendingundersized or oversized records.

The capabilities of the present invention can be implemented insoftware, firmware, hardware or some combination thereof.

As will be appreciated by one skilled in the art, the present inventionmay be embodied as a system, method or computer program product.Accordingly, the present invention may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,the present invention may take the form of a computer program productembodied in any tangible medium of expression having computer usableprogram code embodied in the medium. An example includes computerprogram product 1200 as depicted in FIG. 12 on a computer usable medium1202 with computer program code logic 1204 containing instructionsembodied in tangible media as an article of manufacture. There may bemultiple computer program products 1200, with each directed to implementfunctional processes on separate processing circuitry. For example, theprocesses 1000 and 1100 of FIGS. 10 and 11 can be embodied as computerprogram code logic 1204 on separate computer program products 1200, withone executable on the host system 101 of FIG. 1 and the other executableat one or more control units 110 of FIG. 1. Alternatively, the processes1000 and 1100 can be stored as computer executable code on a singlecomputer program product 1200.

Any combination of one or more computer usable or computer readablemedium(s) may be utilized. The computer-usable or computer-readablemedium may be, for example but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, or semiconductor system, apparatus,device, or propagation medium. More specific examples (a non-exhaustivelist) of the computer-readable medium would include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CDROM), an optical storage device, a transmission media such as thosesupporting the Internet or an intranet, or a magnetic storage device.Note that the computer-usable or computer-readable medium could even bepaper or another suitable medium upon which the program is printed, asthe program can be electronically captured, via, for instance, opticalscanning of the paper or other medium, then compiled, interpreted, orotherwise processed in a suitable manner, if necessary, and then storedin a computer memory. In the context of this document, a computer-usableor computer-readable medium may be any medium that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer-usable medium may include a propagated data signal with thecomputer-usable program code embodied therewith, either in baseband oras part of a carrier wave. The computer usable program code may betransmitted using any appropriate medium, including but not limited towireless, wireline, optical fiber cable, RF, etc.

Computer program code for carrying out operations of the presentinvention may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The computer program code logic 1204 of FIG. 12 represents anembodiment of program code. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

The present invention is described with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products according to embodiments of the invention. Itwill be understood that each block of the flowchart illustrations and/orblock diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide processes for implementing the functions/actsspecified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneore more other features, integers, steps, operations, elementcomponents, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A computer program product for processing a chained-pair linkedtransport control channel program at a control unit configured forcommunication with an input/output (I/O) subsystem in an I/O processingsystem, the computer program product comprising: a tangible storagemedium readable by a processing circuit and storing instructions forexecution by the processing circuit for performing a method comprising:receiving a first command message at the control unit from the I/Osubsystem, the first command message specifying that a device commandword (DCW) list is encoded in a data message associated with the firstcommand message as part of the chained-pair linked transport controlchannel program; receiving a second command message at the control unitfrom the I/O subsystem chained-pair linked to the first command message,the second command message specifying data attributes associated withthe DCW list; extracting the DCW list from the data message in responseto receiving the data message; and executing the DCW list.
 2. Thecomputer program product of claim 1 wherein the method furthercomprises: determining that the first command message specifies the DCWlist is encoded in the data message in response to examining a firstcontrol command and a first data flag in the first command message;determining that the second command message specifies data attributesassociated with the DCW list in response to examining a second controlcommand and a second data flag in the second command message; andestablishing that the first and second command messages are parts of thechained-pair in response to a common sequence number in the first andsecond command messages.
 3. The computer program product of claim 1wherein the method further comprises: receiving a second data messageassociated with the second command message, the second data messageincluding customer data for the DCW list; transmitting a responsemessage without extended status in response to receiving the DCW list;and transmitting a response message with extended status in response toexecuting the DCW list.
 4. The computer program product of claim 1wherein the method further comprises: executing a read command;determining that a record of data associated with the read command issmaller than a data count value; checking a pad flag associated with theread command; and inserting pad data in a data message in response toassertion of the pad flag, wherein the pad data is inserted up to thedata count value.
 5. The computer program product of claim 1 wherein themethod further comprises: comparing a record length relative to a datacount value; checking a suppress incorrect length flag; and suppressingidentification of an incorrect length condition in response to assertionof the suppress incorrect length flag when the record length does notmatch the data count value.
 6. The computer program product of claim 1wherein the method further comprises: determining that one or morecommands associated with a communication exchange cannot execute;sending a termination status message to the I/O subsystem indicating aninability to execute; and closing open communication exchanges withsequence numbers greater than a sequence number associated with the oneor more non-executable commands.
 7. The computer program product ofclaim 1 wherein the method further comprises: receiving a login requestmessage; and transmitting a login accept message in response to thelogin request message, the login accept message including a maximumlinked commands field indicating a maximum count of additional commandmessages queued after the first command message that the control unitsupports, wherein the first and second command messages are counted as asingle chained-pair message.
 8. The computer program product of claim 1wherein the I/O subsystem is a channel subsystem and the first andsecond command messages are transport command information unitscomprising transport command control blocks (TCCBs) with one or morecommands in DCWs of the first command message and a single DCW in thesecond message specifying the data attributes.
 9. An apparatus forprocessing a chained-pair linked transport control channel program at acontrol unit in an I/O processing system, the apparatus comprising: acontrol unit configured for communication with an I/O subsystem of theI/O processing system, the control unit performing a method comprising:receiving a first command message from the I/O subsystem, the firstcommand message specifying that a device command word (DCW) list isencoded in a data message associated with the first command message aspart of the chained-pair linked transport control channel program;receiving a second command message from the I/O subsystem chained-pairlinked to the first command message, the second command messagespecifying data attributes associated with the DCW list; extracting theDCW list from the data message in response to receiving the datamessage; and executing the DCW list.
 10. The apparatus of claim 9wherein the method further comprises: determining that the first commandmessage specifies the DCW list is encoded in the data message inresponse to examining a first control command and a first data flag inthe first command message; determining that the second command messagespecifies data attributes associated with the DCW list in response toexamining a second control command and a second data flag in the secondcommand message; and establishing that the first and second commandmessages are parts of the chained-pair in response to a common sequencenumber in the first and second command messages.
 11. The apparatus ofclaim 9 wherein the method further comprises: receiving a second datamessage associated with the second command message, the second datamessage including customer data for the DCW list; transmitting aresponse message without extended status in response to receiving theDCW list; and transmitting a response message with extended status inresponse to executing the DCW list.
 12. The apparatus of claim 9 whereinthe method further comprises: executing a read command; determining thata record of data associated with the read command is smaller than a datacount value; checking a pad flag associated with the read command; andinserting pad data in a data message in response to assertion of the padflag, wherein the pad data is inserted up to the data count value. 13.The apparatus of claim 9 wherein the method further comprises: comparinga record length relative to a data count value; checking a suppressincorrect length flag; and suppressing identification of an incorrectlength condition in response to assertion of the suppress incorrectlength flag when the record length does not match the data count value.14. The apparatus of claim 9 wherein the method further comprises:determining that one or more commands associated with a communicationexchange cannot execute; sending a termination status message to the I/Osubsystem indicating an inability to execute; and closing opencommunication exchanges with sequence numbers greater than a sequencenumber associated with the one or more non-executable commands.
 15. Theapparatus of claim 9 wherein the method further comprises: receiving alogin request message; and transmitting a login accept message inresponse to the login request message, the login accept messageincluding a maximum linked commands field indicating a maximum count ofadditional command messages queued after the first command message thatthe control unit supports, wherein the first and second command messagesare counted as a single chained-pair message.
 16. The apparatus of claim9 wherein the I/O subsystem is a channel subsystem and the first andsecond command messages are transport command information unitscomprising transport command control blocks (TCCBs) with one or morecommands in DCWs of the first command message and a single DCW in thesecond message specifying the data attributes.
 17. A method forprocessing a chained-pair linked transport control channel program at acontrol unit configured for communication with an input/output (I/O)subsystem in an I/O processing system, the method comprising: receivinga first command message at the control unit from the I/O subsystem, thefirst command message specifying that a device command word (DCW) listis encoded in a data message associated with the first command messageas part of the chained-pair linked transport control channel program;receiving a second command message at the control unit from the I/Osubsystem chained-pair linked to the first command message, the secondcommand message specifying data attributes associated with the DCW list;extracting the DCW list from the data message in response to receivingthe data message; and executing the DCW list.
 18. The method of claim 17further comprising: determining that the first command message specifiesthe DCW list is encoded in the data message in response to examining afirst control command and a first data flag in the first commandmessage; determining that the second command message specifies dataattributes associated with the DCW list in response to examining asecond control command and a second data flag in the second commandmessage; and establishing that the first and second command messages areparts of the chained-pair in response to a common sequence number in thefirst and second command messages.
 19. The method of claim 17 furthercomprising: receiving a second data message associated with the secondcommand message, the second data message including customer data for theDCW list; transmitting a response message without extended status inresponse to receiving the DCW list; and transmitting a response messagewith extended status in response to executing the DCW list.
 20. Acomputer program product for processing a chained-pair linked transportcontrol channel program at a channel subsystem configured forcommunication with a control unit in an I/O processing system, thecomputer program product comprising: a tangible storage medium readableby a processing circuit and storing instructions for execution by theprocessing circuit for performing a method comprising: configuring afirst command message specifying that a device command word (DCW) listis encoded in a data message associated with the first command messageas part of the chained-pair linked transport control channel program;configuring a second command message chained-pair linked to the firstcommand message, the second command message specifying data attributesassociated with the DCW list; transmitting the first and second commandmessages from the channel subsystem to the control unit; andtransmitting the DCW list from the channel subsystem to the control unitin the data message.
 21. The computer program product of claim 20wherein a first sequence number associated with the first commandmessage is written to the second command message as a second sequencenumber in response to a companion indicator associated with the firstcommand message, indicating the chained-pair link between the first andsecond command messages.
 22. The computer program product of claim 20wherein the method further comprises: setting a first control commandand a first data flag in the first command message to inform the controlunit that the first command message specifies the DCW list is encoded inthe data message; and setting a second control command and a second dataflag in the second command message to inform the control unit that thesecond command message specifies the data attributes associated with theDCW list.
 23. The computer program product of claim 20 wherein themethod further comprises: setting a pad flag associated with one or moreof the commands transmitted to the control unit requesting that pad datais inserted up to a data count value; and setting a suppress incorrectlength flag to request suppressing identification of an incorrect lengthcondition in response to assertion of the suppress incorrect length flagwhen a record length on a read or write operation at the control unitdoes not match the data count value.
 24. The computer program product ofclaim 20 wherein the method further comprises: transmitting a loginrequest message including a channel maximum linked commands fieldindicating a maximum count of additional command messages queued afterthe first command message that the channel subsystem supports; receivinga login accept message in response to the login request message, thelogin accept message including a control unit maximum linked commandsfield indicating a maximum count of additional command messages queuedafter the first command message that the control unit supports; andlimiting transmission of command messages to the control unit to complywith the control unit maximum linked commands field.
 25. An apparatusfor processing a chained-pair linked transport control channel programat a channel subsystem configured for communication with a control unitin an I/O processing system, the apparatus comprising: a channelsubsystem configured for communication with a control unit of the I/Oprocessing system, the channel subsystem performing a method comprising:configuring a first command message specifying that a device commandword (DCW) list is encoded in a data message associated with the firstcommand message as part of the chained-pair linked transport controlchannel program; configuring a second command message chained-pairlinked to the first command message, the second command messagespecifying data attributes associated with the DCW list; transmittingthe first and second command messages from the channel subsystem to thecontrol unit; and transmitting the DCW list from the channel subsystemto the control unit in the data message.